SAN ANTONIO — Black women who exercised vigorously for at least 3 hours per week on a regular basis had a 47% reduced risk of developing estrogen-receptor (ER)-negative breast cancer, compared with those who exercised an average of 1 hour per week, according to results from the Black Women's Health Study (BWHS).
"These findings are very encouraging," BWHS coleader Lucile Adams-Campbell, PhD, professor of oncology and associate director of minority health and health disparities research at the Georgetown Lombardi Comprehensive Cancer Center in Washington, DC, said in a statement.
"Knowing that exercise may protect against breast cancers that disproportionately strike black women is of great public health importance," she explained.
ER-negative breast cancers, which include HER2-positive and triple-negative tumors, have a worse prognoses than ER-positive tumors.
It is not known why this more aggressive form of breast cancer occurs more frequently and is linked to higher mortality rates in black than in white women.
The analysis, which was presented here at the 36th Annual San Antonio Breast Cancer Symposium, involved 44,704 women 30 years or older from the overall 59,000 women who participated in the BWHS.
The study is the first to look at exercise and molecular subtypes in black women.
During the 20-year observational study, the degree of vigorous exercise was assessed when participants were in high school, and when they were around 21, 30, and 40 years of age.
The researchers used less than 1 hour per week as a reference, and found that increased amounts of exercise corresponded to a decreased estimated incidence rate ratio (IRR) of ER-negative, but not ER-positive, breast cancer.
The IRR is a relative measure used to compare the incidence rates of events occurring at any given point in time.
Overall, a lifetime average of 3 or more hours of vigorous activity per week (across all 4 time points) was associated with a lower IRR for ER-negative cancer (0.53; 95% confidence interval [CI], 0.31 - 0.90), relative to a lifetime average of less than 1 hour per week. But in these enthusiastic exercisers, the same effect was not found for ER-positive breast cancer (IRR, 0.95).
The inverse association between average vigorous exercise and ER-negative cancer incidence (the greater the exercise hours, the lower the incidence) was most apparent in nonobese women (IRR, 0.50; 95% CI, 0.26 - 0.95) and postmenopausal women (IRR, 0.41; 95% CI, 0.17 - 0.98).
However, when the researchers looked more at a specific point in time (age 30), they found that there was a trend toward decreasing risk with increasing hours of weekly exercise, although the numbers did not reach significance.
The only statistically significant finding showed a risk reduction in 30-year-old women who exercised 7 or more hours per week (IRR, 0.66; 95% CI, 0.36 - 0.92).
This study strengthens the evidence that vigorous exercise reduces the occurrence of ER-negative breast cancer, the study authors note.
"We all want to do what we can to reduce our risk of disease and improve our health, and, along with other well-known benefits, we now show that exercise can possibly stave off the development of potentially lethal breast cancer in black women," said Dr. Adams-Campbell.
The results are also important for nonblack women because of the general association between ER-negative cancer and high mortality, she added.
Although exercise has been associated with a reduced risk for breast cancer in general, evidence looking specifically at black women and breast cancer subtypes is sparse.
In fact, a recent multiracial analysis of the Southern Community Cohort Study demonstrated that greater physical activity was associated with reduced odds for breast cancer only in white women (P trend = .03) (Cancer Prev Res. 2013;6:566-576).
The Black Women's Health Study is funded by the National Cancer Institute. The authors have disclosed no relevant financial relationships.
36th Annual San Antonio Breast Cancer Symposium (SABCS): Abstract PD2-7. Presented December 11, 2013.
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Cite this: Exercise Cuts Risk for Aggressive Breast Cancers in Blacks - Medscape - Dec 18, 2013.